Minimization of metal sulphides bioleaching from mine wastes into the aquatic environment

The continuous presence of toxic elements in the aquatic environments around mine tailings occurs due to bioleaching or chemical extraction promoted by the mining operations. Biogenic passivation treatment of tailings dams can be a new environment-friendly technique to inhibit the solubility of heavy metals. In spite of current bioleaching researches, we tried to minimize the mobility of the trace elements in the laboratory scale through the formation of a passivation layer in the presence of a mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. The X-ray diffraction (XRD) and scanning electron microscope (SEM) represented the jarosite generation as an inhibitory layer on the mineral surfaces of the tested materials. More detailed observations on electron probe micro-analyzer (EPMA) showed the co-precipitation of metals with the passivation layer. Thereby, the passivation layer demonstrates potential in elements immobilization which, in turn, can be optimized in the natural systems. Our working hypothesis was to exploit and optimize the formation of the passivation layer to maximize the immobilization of heavy metals (e.g., Cu, Cr). The optimization process of bioleaching experiments using indigenous bacteria caused a reduced solubility for Cu (from around 20% to 4.5%) and Cr (from around 30% to 10.6%) and the formation of 6.5 gr passivation layer. The analyses finally represented the high efficiency of the passivation technique to minimize metals bioleaching in comparison to earlier studies

Electrochemical and reactions mechanisms in the minimization of toxic elements transfer from mine-wastes into the ecosystem

The generation of self-protective mine-wastes through a superficial secondary layer to prohibit the leaching of toxic elements can be a new perspective for future environmental studies. The bioleaching-based treatment can lead to the surface passivation of contaminated minerals, which inhibit trace elements mobility. In this work, the electrochemical and passivation mechanisms for the minimization of mine-wastes dissolution were studied on a laboratory scale. The electrochemical behavior of bio-treated soil during surface passivation was investigated by cyclic voltammetry (CV) analysis. The concentration of Fe2+ and Fe3+ in bio-treatment leachates was analyzed to improve our knowledge about the competitive effect of iron ions on the chemical and bacterial dissolution of sulfide tailings. The results of transmission electron microscopy (TEM) and electron probe micro-analyzer (EPMA) confirmed the surface coating of metal sulfides, which led to approximately complete passivation of the minerals. The CV analysis represented that the passivation layer produced in the presence of Acidithiobacillus bacteria was stable in a wide range of redox potential. This study showed that Fe3+ ions play a controlling role in the dissolution process. The high concentration of ferric ions generates a passivation layer in the bulk solution of (bio)leaching. The kinetics study of copper mobility in the minimization process conformed to diffusion control. The results of the kinetics analysis showed that the Cr bioleaching mechanisms followed both the chemical model and diffusion model

Effect of biogenic jarosite on the bio-immobilization of toxic elements from sulfide tailings

The discharge of toxic elements from tailings soils in the aquatic environments occurs chiefly in the presence of indigenous bacteria. The biotic components may interact in the opposite direction, leading to the formation of a passivation layer, which can inhibit the solubility of the elements. In this work, the influence of jarosite on the bio-immobilization of toxic elements was studied by native bacteria. In batch experiments, the bio-immobilization of heavy metals by an inhibitory layer was examined in the different aquatic media using pure cultures of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. A variety of analyses also investigated the mechanisms of metals bio-immobilization. Among different tests, the highest metal solubility yielded 99% Mn, 91% Cr, 95% Fe, and 78% Cu using A. ferrooxidans in 9KFe medium after ten days. After 22 days, these percentages decreased down to 30% Mn and about 20% Cr, Fe, and Cu, likely due to metal immobilization by biogenic jarosite. The formation of jarosite was confirmed by an electron probe micro-analyzer (EPMA), X-ray diffraction (XRD), and scanning electron microscope (SEM). The mechanisms of metal bio-immobilization by biogenic jarosite from tailings soil confirmed three main steps: 1) the dissolution of metal sulfides in the presence of Acidithiobacillus bacteria; 2) the nucleation of jarosite on the surface of sulfide minerals; 3) the co-precipitation of dissolved elements with jarosite during the bio-immobilization process, demonstrated by a structural study for jarosite. Covering the surface of soils by the jarosite provided a stable compound in the acidic environment of mine-waste